PWM (Pulse Width Modulation) inverters are commonly used to convert DC power into AC power. Typical applications of these inverters include use in uninterruptible power supplies (UPS), fuel cells, photovoltaic panels, and wind turbines. Further, PWM inverters may be used to provide compensation for reactive loads, harmonic cancellation of supply grids, or as variable-speed drives for induction motors. The most commonly used inverters are two-level inverters and three-level inverters.
Two-level inverters produce a modulated output having two fixed voltage levels. While potentially low cost, there are some drawbacks associated with using two-level inverters. First, the voltage swing of the inverter transistors is equal to the full, applied DC rail voltage of the inverter. This voltage swing can cause significant switching loss in the inverter transistors. Stated differently, switching loss of an inverter transistor is proportional to the amplitude of the voltage swing. To compensate for these switching losses, a lower PWM frequency may be chosen. However, this frequency may be so low that it creates audible noise (<20 kHz) or excessive output ripple current. A further drawback of two-level inverters is that the voltage output is typically derived directly from the switching bridge and may have a very high content of high frequency harmonics. This may cause additional losses in the output filter when used in typical applications. A typical two-level inverter 100 having switching transistors 101 and 102 and an output LC filter is shown in FIG. 1.
Three-level inverters produce a modulated output consisting of three fixed voltage levels. For a given output voltage this results in a lower voltage swing across the transistors than in the two-level inverters discussed above. As a result, three-level inverters produce fewer high-frequency voltage harmonics (i.e. significant amounts of energy at frequencies that are multiples of the switching frequency), allowing one to use smaller and cheaper output filter chokes (i.e. the inductive element of the output filter used to isolate the output alternating current from the output of the inverter). The reduced voltage swing and switching losses characteristic of three-level inverters make these inverters generally more efficient than two-level inverters. However, three-level inverters are typically more complex and expensive than two-level inverters. A typical three-level inverter 200 is shown in FIG. 2. The inverter 200 includes switching transistors 201, 202, 203 and 204, includes coupling diodes 211 and 212, and also includes an output LC filter.
The most common switching elements used in inverter designs are field effect transistors (FET), such as metal oxide field effect transistors (MOSFET), bipolar transistors, such as insulated gate bipolar transistors (IGBT) and bipolar junction transistors (BJT), and gate turn-off thyristors (GTO.) Traditionally, MOSFETs have been used for low DC voltage or low power inverter designs; IGBTs have been used in medium to high power or high voltage inverter designs; and GTOs have been used in very high power inverter designs.
To obtain low losses in an inverter, it is desirable to use transistors that have low switching losses and to use anti-parallel/freewheeling diodes (e.g. 311) across each transistor with good recovery behavior. MOSFETs are generally known to have very good switching performance, but the internal (anti-parallel) body-diode exhibits poor recovery behavior. Poor recovery behavior of freewheeling diodes will produce undesirable effects such as high peak currents and/or oscillations when rapidly commutated (turned off). To compensate, inverter designs using MOSFETs have traditionally required the addition of both series and freewheeling ultra-fast diodes. The addition of these diodes significantly increases the cost of the inverter design and adds conduction losses. For this reason, IGBTs have been a more practical choice for inverters operating above 100-200 VDC. IGBTs typically have poorer switching performance than MOSFETS, but require the addition of fewer diodes to provide rapid recovery behavior, since the internal series diode present in IGBTs allows the designer to add a single diode to the freewheeling path. The use of IGBTs can reduce the cost of an inverter design but may lower inverter efficiency at higher (non-audible) frequencies. FIG. 3, shows a prior art three-level inverter 300 built using IGBTs. The inverter 300 contains four IGBTs 301, 302, 303, and 304 in series, each coupled to additional diodes 311, 312, 313, and 314 placed anti-parallel in the flyback path to conduct reverse currents. Further diodes 315 and 316 provide paths to ground when the rail IGBTs 301 and 304 are turned off by the control signals 321 and 324. Control signals 321, 322, 323, and 324 control the states of the IGBTs 301, 302, 303, and 304 to regulate the output voltage 337 of the LC filter placed before the load. Another important aspect to inverter design is the ability to handle different types of loads: resistive loads, reactive loads, such as inductive or capacitive loads, and non-linear loads. The use of an inductive or a capacitive load with an inverter can result in phase shifts between the output voltage and the output current. The phase shift between current and voltage is often described using four distinct phase quadrants. FIG. 4 shows a graph 400 of a typical inverter output current waveform 401 and an output voltage waveform 402. A first quadrant 410 of the graph is characterized by both positive current and voltage values; a second quadrant 420 is characterized by positive current and negative voltage values; a third quadrant 430 is characterized by both negative current and voltage values; and a fourth quadrant 440 is characterized by positive voltage and negative current values. Both 2- and 3-level inverters are typically required to operate in all four quadrants of phase shown in the graph 400 to be practical for universal application with different types of loads. This requires the inverter to successfully operate with instantaneous output voltage and current having the same (first and third quadrant, 410 and 430) or opposite (second and fourth quadrants, 420 and 440) polarity.
As stated, a typical application for a three-level inverter is a DC-to-AC converter in a UPS to convert energy stored in a storage element, such as a battery, to AC power for loads requiring AC power. For a typical UPS application, the inverter may be used mainly to drive resistive or partly reactive (both capacitive and inductive) loads, and must also support non-linear loads. A typical UPS load will require no more than a 45-degree load current phase shift from load voltage. This corresponds to a power factor (i.e. the cosine of the phase angle between the voltage and current, providing a measure of load reaction) of approximately 0.7. Even with this low power factor, an inverter will still typically operate in the first and third quadrants for the majority of time.